The invention is described hereunder with respect to polishing semiconductor wafers. In preparing such wafers for the various masking, doping and diffusion steps used in the manufacture of semiconductor devices, each wafer is polished on one of its two major surfaces to a highly reflective and extremely flat surface condition. Conventional semiconductor forming steps are subsequently applied to the polished surface. Experience has shown that any imperfections greater than 0.5 mils (thousandths of an inch) and in some cases even as small as 0.2 mils from a flat plane may cause finished semiconductor devices to be defective. It is, therefore, desirable to polish the "active" side or "frontside" of the wafer to a flatness having no more than about 0.2 to 0.5 mils nonlinear thickness variation (NTV) from a perfect plane. The reverse or "backside" of the wafer typically receives no polishing treatment.
Various polishing machines are commercially available which can be used to polish the wafers to the required flatness. These machines use one or more wafer carriers. A wafer carrier provides a flat circular surface to which one or more wafers are removably mounted. The carrier is then inverted upon and forced against a rotatable turntable. The turntable provides a flat polishing surface which is powered to move with respect to the wafers on the carrier. The carrier also rotates. Some types of polishing machines use an additional translational movement of the carrier back and forth across the rotating turntable to enhance the randomness of the polishing motion.
The relative movement between the rotating polishing surface of the turntable and the rotating carrier under a given force or pressure between them polishes the wafers. Abrasive or chemical slurries are added to sustain the polishing action on the wafers. Water may be added for cooling and for cleaning the wafers.
A continuous problem in polishing wafers has been to mount the wafers on the carriers to retain them securely against lateral or shear forces generated by the movement of the polishing surface against the wafers. The wafers must be mounted perfectly flat without introducing a bending stress. If such a stress does exist after the wafer is mounted and the wafer is polished flat, a release of the wafer from the carrier relieves the stress and introduces a new surface deviation into the polished surface of the wafer, thereby destroying its flatness.
Prior art techniques of mounting the wafers to the carrier by a film of wax tend to introduce such surface deviations or waviness in polished wafers, especially in presently preferred, larger diameter wafers. Such prior art wax mounting techniques also tend to leave wax residues on the wafers. The removal of such residues from the wafers is cumbersome and costly.
In seeking alternatives to wax mounting techniques, various types of tacky layers have been used to cover the carrier for mounting the wafers. Soft layers also tend to introduce a waviness into the wafers. Harder layers as, for instance, cellular polyurethane films do not offer sufficient lateral restraint to retain the wafers securely while they are being polished. Such harder layers have, however, been used together with lateral mechanical restraints.
The mechanical restraints tend to introduce problems as well. Pin restraints may chip the edges of the wafers. Wafer pockets provided by templates may accumulate slurries which can affect the ultimate flatness of the wafer. While mechanical lateral restraints are commonly used with presently known polishing techniques, their use requires a great deal of caution.
A composite sheet of a layered structure is commercially available as a mounting surface for the wafers. The sheet has a fibrous resiliently compressible underlayer and a relatively hard but flexible microcellular top layer. The top layer is of polyurethane. One such sheet is sold by the George Newman Co. under the designation FP-5G.
When the sheet is attached to the carrier and the sheet is wetted, wafers are pressure-mounted to the sheet. The wafers are then polished in the described manner without any further lateral restraint on the wafers. However, for the sheet to retain its initial mounting characteristics, it appears that it has to be roughened after being used and that the wafers have to be mounted with a relatively large mounting force. Applying such a mounting force is undesirable. It may damage the wafers and can introduce into the wafers the undesirable stresses which cause waviness in the polished wafers. It is, therefore, highly desirable to improve the mounting characteristics of such a sheet to permit the wafers to be mounted thereto without exerting a large mounting force to press the wafers against the sheet.